Chapter 8 Alkenes and Alkynes II: Addition Reactions

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Chapter 8Alkenes and Alkynes II Addition
Reactions
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• Introduction Additions to Alkenes
• Generally the reaction is exothermic because one
  p and one s bond are converted to two s bonds
• The p electrons of the double bond are loosely
  held and are a source of electron density, i.e.
  they are nucleophilic
• Alkenes react with electrophiles such as H from
  a hydrogen halide to form a carbocation

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• The carbocation produced is an electrophile
• It can react with a nucleophile such as a halide
• Insert top scheme pg 331
• In addition reactions the alkene changes from a
  nucleophile in the first step to an electrophile
  in the second

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• Addition of Hydrogen Halides to Alkenes
  Markovnikovs Rule
• Addition of HBr to propene occurs to give
  2-bromopropane as the major product
• Markovnikovs Rule (Original) addition of HX to
  an alkene proceeds so that the hydrogen atom adds
  to the carbon that already has the most hydrogen
  atoms

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• Mechanism for hydrogen halide addition to an
  alkene
• The reaction has a highly endergonic first step
  (rate determining) and a highly exergonic second
  step

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• Theoretical Explanation of Markovnikovs Rule
• The product with the more stable carbocation
  intermediate predominates
• The most stable carbocation is formed fastest
  because it has a lower DG
• The transition state for the rate determining
  step (first step) resembles a carbocation and is
  stabilized by factors which stabilize carbocations

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• Addition of HBr to 2-methylpropene gives only
  tert-butyl bromide
• Modern Statement of Markovnikovs Rule In the
  ionic addition of an unsymmetrical reagent to a
  double bond, the positive portion of the adding
  reagent attaches itself to a carbon atom of the
  double bond so as to yield the more stable
  carbocation as an intermediate
• Regioselective Reaction When a reaction that can
  potentially yield two or more constitutional
  isomers actually produces only one or a
  predominance of one isomer

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• Stereochemistry of the Ionic Addition to an
  Alkene
• Addition of HBr to butene yields a chiral
  molecule
• A racemic mixture is produced because the
  intermediate carbocation is achiral

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• Addition of Sulfuric Acid to Alkenes
• Addition of concentrated sulfuric acid to alkenes
  leads to alkyl hydrogen sulfates which are
  soluble in the acid
• The addition follows Markovnikovs rule
• The sulfate can be hydrolyzed by heating with
  water
• The net result is Markovnikov addition of water
  to an alkene

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• Addition of Water to Alkenes Acid-Catalyzed
  Hydration
• The reaction of alkenes with dilute aqueous acid
  leads to Markovnikov addition of water
• The mechanism is the reverse of that for
  dehydration of an alcohol
• The first step in which a carbocation is formed
  is rate determining

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• The hydration of alkenes and the dehydration of
  alcohols are simply reverse reactions of one
  other
• The reaction is governed by the position of all
  the equilibria
• Hydration is favored by addition of a small
  amount of acid and a large amount of water
• Dehydration is favored by concentrated acid with
  very little water present (removal of water
  produced also helps favor dehydration)
• Carbocation rearrangements can occur

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• Alcohols from Alkenes Through Oxymercuration-Demer
  curation Markovnikov Addition
• The procedure gives high yields of alcohols and
  avoids rearrangements
• The reaction shows Markovnikov selectivity

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• The mechanism involves formation of a bridged
  mercurinium ion

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• Alcohols from Alkenes through Hydroboration-Oxidat
  ion Anti-Markovnikov Syn Hydration
• The reaction leads to syn and anti-Markovnikov
  addition of water to alkenes
• Hydroboration Synthesis of Alkylboranes
• The elements of hydrogen and boron are added
  across the double bond
• In practice, a borane complex with the solvent
  tetrahydrofuran (THF) is often used

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• Mechanism of Hydroboration
• Boron hydride adds successively to three
  molecules of alkene
• Boron becomes attached to the least substituted
  carbon of the double bond
• The bulky boron group can approach the least
  sterically hindered carbon more easily
• This orientation also allows a d charge in the
  transition state to reside at the most
  substituted carbon
• This orientation leads to anti-Markovnikov
  product
• The boron and hydride add with syn
  stereochemistry

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• Oxidation and Hydrolysis of Alkylboranes
• Oxidation and hydrolysis to the alcohol takes
  place with retention of stereochemistry at the
  carbon bonded to boron

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• Hydroboration of methylcyclopentene gives the
  anti-Markovnikov product with syn addition of the
  elements of water
• Summary of Alkene Hydration Methods
• Acid-catalyzed hydrolysis Markovnikov addition
• Oxymercuration Markovnikov addition
• Hydroboration-Oxidation anti-Markovnikov and syn
  addition

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• Addition of Bromine and Chlorine to Alkenes
• Addition produces vicinal dihalides
• This reaction is used as a test for alkenes
  because the red color of the bromine reagent
  disappears when an alkene (or alkyne) is present
• Alkanes do not react with bromine in the dark

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• Mechanism of Halogen Addition
• A bromonium ion intermediate results instead of
  the carbocation seen in other addition reactions

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• Stereochemistry of the addition of Halogens to
  Alkenes
• The net result is anti addition because of SN2
  attack on the bromonium ion intermediate
• When cyclopentene reacts the product is a racemic
  mixture of trans-1,2-dibromocyclopentane
  enantiomers

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• Stereospecific Reactions
• A reaction is stereospecific if a particular
  stereoisomeric form of the starting material
  reacts in such a way that it gives a specific
  stereoisomeric form of the product
• Example cis- and trans-2-butene give
  stereoisomeric products when halogenated
• Halogenation of double bonds is stereospecific

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• Halohydrin Formation
• If halogenation is carried out in aqueous
  solvent, the water molecule can act as a
  nucleophile to open the halonium ion
• The product is a halohydrin

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• In unsymmetrical alkenes, the bromonium ion will
  have some of its d charge density on the most
  substituted of the two carbons
• The most substituted carbon can best accommodate
  d charge
• The water nucleophile will tend to react at the
  carbon with the most d charge

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• Divalent Carbon Compounds Carbenes
• Carbenes have divalent but neutral carbons with a
  lone pair of electrons
• Carbenes are highly reactive
• Structure and Reaction of Methylene
• Methylene can be made by heat or light initiated
  decomposition of diazomethane
• Loss of a molecule of the stable gas nitrogen
  drives this reaction
• Methylene reacts with alkenes to form
  cyclopropanes

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• Reactions of Other Carbenes Dihalocarbenes
• Carbenes add to double bonds in a stereospecific
  manner
• Dihalocarbenes are formed by a elimination of
  compounds such as chloroform
• Carbenoids The Simmons-Smith Cyclopropane
  Synthesis
• A carbene-like species is formed which then
  reacts with alkenes

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• Oxidations of Alkenes Syn 1,2-Dihydroxylation
• Either OsO4 or KMnO4 will give 1,2 diols
  (glycols)
• Mechanism for Syn Hydroxylation of Alkenes
• Cyclic intermediates result from reaction of the
  oxidized metals
• The initial syn addition of the oxygens is
  preserved when the oxygen-metal bonds are cleaved
  and the products are syn diols

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• Oxidative Cleavage of Alkenes
• Reaction of an alkene with hot KMnO4 results in
  cleavage of the double bond and formation of
  highly oxidized carbons
• Unsubstituted carbons become CO2, monosubstituted
  carbons become carboxylates and disubstituted
  carbons become ketones
• This be used as a chemical test for alkenes in
  which the purple color of the KMnO4 disappears
  and forms brown MnO2 residue if alkene (or
  alkyne) is present

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• Solved Problem
• An unknown alkene with formula C7H12 yields only
  the following product on oxidation with hot KMnO4
• Answer Since no carbons are missing in the
  product, the alkene must be part of a ring in the
  original molecule

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• Ozonolysis of Alkenes
• Cleavage of alkenes with ozone and workup with
  zinc in acetic acid leads to less highly oxidized
  carbons than products from cleavage with hot
  KMnO4
• Unsubstituted carbons are oxidized to
  formaldehyde, monosubstituted carbons are
  oxidized to aldehydes and disubstituted carbons
  are oxidized to ketones

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• Ozone adds across the double bond to form the
  initial ozonide which rearranges to a highly
  unstable ozonide
• The ozonides react with zinc and acetic acid to
  effect the cleavage

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• Addition of Bromine and Chlorine to Alkynes
• Addition of halogen to alkynes can occur once or
  twice depending on how many equivalents of the
  halogen are added
• Addition of one equivalent usually proceeds to
  give the trans dihalide

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• Addition of Hydrogen Halides to Alkynes
• Addition of hydrogen halides occurs once or twice
  depending on how many molar equivalent of
  hydrogen halide are added
• Both additions are Markovnikov and give
  gem-halides
• HBr can be generated by reaction of acetyl
  bromide and alumina
• Anti-Markovnikov addition of HBr occurs in the
  presence of peroxide (See Chapter 10)

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• Oxidative Cleavage of Alkynes
• Reaction of alkynes with ozone or basic potassium
  permanganate both lead to formation of carboxylic
  acids

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• Synthetic Strategies Revisited
• Example Synthesis of 1-butene from compounds
  with two or fewer carbons

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• Example Synthesis of (2R, 3R)-2,3-butandiol and
  its enantiomer from compounds of two carbon atoms
  or fewer

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